1. Field of the Invention
The present invention relates to a processing method which improves the characteristics of a NOx sensor, and particularly to a method for improving the followability of a NOx signal relative to a change in a NOx concentration.
2. Description of Related Art
Conventionally, as an apparatus for measuring a NOx concentration in a measurement gas exemplified by a combustion gas or an exhaust gas of an internal combustion engine such as an automobile engine, there has been known a NOx sensor in which a sensor element is formed by using an oxygen-ion conductive solid electrolyte layer such as a zirconia (ZrO2) layer. In this NOx sensor, the concentration of a NOx gas is obtained by utilizing the fact that the amount of oxygen ion generated when a NOx gas is decomposed in a measuring electrode is proportional to a current (also referred to as a NOx signal) flowing in the measuring electrode and a reference electrode.
This NOx sensor is required to have a good responsiveness (followability) to a change in the NOx concentration. For example, it is demanded that, when the NOx concentration in the measurement gas steeply changes, the value of the NOx signal can promptly follow it and change. On the other hand, even when the concentrations of other gas components in the exhaust gas vary, the NOx signal has to be constant unless the NOx concentration varies. Techniques for improving the followability of the NOx signal are already known (see International Publication WO2008/038773, for example).
International Publication WO 2008/038773 discloses that a sensor element constituted of the above-mentioned solid electrolyte and included in a NOx sensor is heated at a temperature of 500° C. or higher and for 15 minutes or longer in an atmosphere containing hydrocarbon (HC) and having an air ratio of 0.80 to 1.10, to thereby resolve an excessive change in an output of a NOx signal.
However, performing a heat treatment on the sensor element in this manner causes the problem that unburned carbon left in the atmosphere adheres to a surface of the element to make the appearance bad, or adversely affects an initial output signal of the sensor element.